Understanding of the mechanisms of melanocytic transformation is a continuing challenge. An in- depth knowledge of the genetic controls of cellular proliferation and cell division may provide critical information regarding the conversion of a normal melanocyte to its neoplastic phenotype. Polo-like kinases (Plks) are a family of highly conserved mitotic serine/threonine kinases that have been shown to play critical roles in cell division and cycle progression. Plk1 is the most widely studied member of the Plk family that contributes to multiple mitotic processes including the activation of the Cdk1/Cyclin B1 cascade, centrosome maturation, bipolar spindle formation and the regulation of Emi1 degradation at mitotic exit. Plk1 has been shown to be over-expressed in several cancers and a forced over-expression of Plk1 in normal cells has been found to cause a transformed phenotype and increased tumorigenicity. However, the functional role of Plk1 in melanocytic transformation is not well understood. In a recent study (J Invest Dermatol, In Press; Appendix-1), we have found that, i) Plk1 was over-expressed in both clinical tissue specimens and cultured human melanoma cells when compared to normal skin tissues and cultured normal melanocytes, respectively; and ii) genetic as well as chemical inhibition of Plk1 resulted in a significant decrease in the viability and growth of melanoma cells without affecting the normal human melanocytes. This study suggested that Plk1 may have an important role in melanoma survival and/or progression. However, additional studies are needed to determine the functional significance of Plk1 in melanocytic transformation and to define its molecular mechanism(s). In this study, we propose to challenge our novel hypothesis that Plk1 plays a critical role in melanocytic transformation and melanoma survival through modulation of Notch signaling. To test the hypothesis, the following specific aims are proposed: 1) To determine the expression pattern of Plk1, Notch and Numb during melanocytic transformation and melanoma development; 2) To determine the involvement of Notch signaling as a downstream target of Plk1 in normal melanocytes and melanoma cells; 3) To define the mechanism by which Plk1 regulates Notch1 signaling; and 4) To determine the functional and therapeutic significance of Plk1 in vivo in athymic nude mice xenografts. The outcome of proposed studies may define i) the role of Plk1 in melanocytic transformation and melanoma progression, ii) a novel connection between a key mitotic regulator (Plk1) with an important signaling pathway (Notch) involved in multiple cellular processes, including melanoma development.